It is well known to make high gloss compositions from rubber modified vinyl aromatic polymers, such as for example high impact polystyrene. A drawback of such compositions, however, is that they are very susceptible to stress cracking when exposed to environmental oils, fats, detergents, or cleaners. Thus the known compositions are deficient in the property of environmental stress crack resistance, even though they have good gloss properties.
The environmental stress crack resistance of such compositions has been addressed in the prior art. For example, in U.S. Pat. No. 5,543,461, the environmental stress crack resistance of impact modified styrenic polymers was increased by increasing the particle size of the impact modifier, and in U.S. Pat. No. 4,144,204, high impact polystyrene compositions with increased environmental stress crack resistance were produced wherein the rubber particles had a diameter of at least 4 microns. In U.S. Pat. No. 4,939,207, blends of a vinyl aromatic polymer, a polyolefin, and a vinyl aromatic monomer conjugated diene star block copolymer were provided which had good chemical resistance. However, in all of the above prior art examples, the high gloss properties of the resulting compositions are lost.
As a result, the known compositions do not provide high impact polystyrene compositions exhibiting a combination of both high gloss and desirable environmental stress crack resistance. Such compositions would be particularly advantageous for thermoforming or sheet extrusion as a gloss cap layer on co-extruded sheet for food packaging or refrigerator liner applications, or for example as molded articles such as telephone handsets subject to attack by the natural oils in the skin.